Encyclopedia of Computer Science and Technology

214 graphics formatsof objects so they look as if they have three dimensions,appearing in proper perspective, with regard to what objectsare behind other objects, and with realistic lighting andshading (see computer graphics).Traditionally, all of the work of producing the actualscreen data was undertaken by the PC’s main processor,executing instructions from the application program anddisplay driver. By putting a separate processor on the videocard (called a video accelerator), together with its own supplyof memory (now up to 256 MB), the main system wasfreed from this burden. A new high-bandwidth connectionbetween the PC motherboard and the graphics card becameavailable with the development of the AGP (AcceleratedGraphics Port). (See bus.) Memory used on video cards isalso optimized for video operations, such as by using typesof memory such as Video RAM (VRAM) that do not need tobe refreshed as frequently.Increasingly, the algorithms for creating realistic images(such as lighting, shading, and texture mapping) are nowsupported by the software built into the video card. Ofcourse, the applications program needs a way to tell thegraphics routines what to draw and how to draw it. Insystems running Microsoft Windows, a program functionlibrary called Direct3D (part of a suite called DirectX) hasbecome the standard interface between applications andgraphics hardware. Video card manufacturers in turn haveoptimized their cards to carry out the kinds of operationsimplemented in DirectX. (A nonproprietary standard calledOpenGL has also achieved some acceptance, particularly onnon-Windows systems.)In evaluating video cards, the tradeoff is between theextent to which advanced graphic features are supportedand the number of frames per second that can be calculatedand sent to the display. If the processing becomes too complicated,the frame rate will slow down and the display willappear to be jerky instead of smooth.Further Reading“Graphics and Displays.” Tom’s Hardware. Available online. URL:http://www.tomshardware.com/graphics/index.html. AccessedAugust 5, 2007.Jones, Wendy. Beginning DirectX 10 Game Programming. Boston:Course Technology, 2007.Luna, Frank. Introduction to 3D Game Programming with DirectX9.0c: A Shader Approach. Plano, Tex.: Wordware Publishing,2006.Sanchez, Julio, and Maria P. Canton. The PC Graphics Handbook.Boca Raton, Fla.: CRC Press, 2003.Shreiner, Dave, et al. OpenGL Programming Guide. 5th ed. UpperSaddle River, N.J.: Addison-Wesley Professional, 2005.“Video Cards.” PC Guide. Available online. URL: http://www.pcguide.com/ref/video/index.htm. Accessed August 5, 2007.graphics formatsBroadly speaking, a graphics file consists of data that specifiesthe color of each pixel (dot) in an image. Since there aremany ways this information can be organized, there are avariety of graphics file formats. The most important andwidely used ones are summarized below.BMP (Windows Bitmap)In a bitmap format there is a group of bits (i.e. a binaryvalue) that specifies the color of each pixel. Windows providesstandard bitmap (BMP) formats for 1-bit (2 colorsor monochrome), 4-bit (16 colors), 8-bit (256 colors), or24-bit (16 million colors). The Windows bitmap format isalso called a DIB (device-independent bitmap) because thestored colors are independent of the output device to beused (such as a monitor or printer). The relevant devicedriver is responsible for translating the color to one actuallyused by the device. Because it is “native” to Windows, BMPis widely used, especially for program graphics resources.Bitmap formats have the advantage of storing the exactcolor of every pixel without losing any information. However,this means that the files can be very large (fromhundreds of thousands of bytes to several megabytes forWindows screen graphics). BMP and other bitmap formatsdo support a simple method of compression called runlengthencoding (RLE), where a series of identical pixels isreplaced by a single pixel and a count. Bitmap files can befurther compressed through the use of utilities such as thepopular Zip program (see data compression).EPSEPS (Encapsulated PostScript) is a vector-based rather thanbitmap (raster) format. This means that an EPS file consistsnot of the actual pixel values of an image, but the instructionsfor drawing the image (including coordinates, colors,and so on). The instructions are specified as a text file inthe versatile PostScript page description language. This formatis usually used for printing, and requires a printer thatsupports PostScript (there are also PostScript renderers thatrun entirely in software, but they tend to be slow and somewhatunreliable).GIFGIF, or Graphics Interchange Format, is a bitmapped formatpromulgated by CompuServe. Instead of reserving enoughspace to store a large number of colors in each pixel, thisformat uses a color table that can hold up to 256 colors.Each pixel contains a reference (index into) the color table.This means that GIF works best with images that have relativelyfew colors and for applications (such as Web pages)where compactness is important. GIF also uses compressionto achieve compactness, but unlike the case with JPEGit is a lossless compression called LZW. There is also a GIFformat that stores simple animations.JPEGJPEG, which stands for Joint Photographic Experts Group,is widely used for digital cameras because of its ability tohighly compress the data in a color graphics image, allowinga reasonable number of high-resolution pictures to bestored in the camera’s onboard memory. The compressionis “lossy,” meaning that information is lost during compression(see data compression). At relatively low compressionratios (such as 10:1, or 10 percent of the original image size)changes in the image due to data loss are unlikely to be